Receivers for digital communications systems regularly check the status of a communications channel in order to detect the presence of a packet on the channel. If the receivers do not check frequently enough, they may miss the beginning of a packet. This is important since control information is often transmitted at the beginning of a packet. However, if the receiver checks the channel too often, then power and processing time is expended unnecessarily on the detection of a packet. Power consumption is a vital consideration for both line-powered and battery-powered devices since power consumption has a direct bearing on power supply size and/or battery life.
A commonly used technique for detecting the presence of a packet on a communications medium involves sampling the communications channel at a sampling rate that is equal to the sampling rate used by the receiver for subsequent processing. In other words, the sampling of the communications channel while attempting to detect the presence of a packet is at the same rate as the sampling of the communications channel while receiving a packet. Using the same sampling rate simplifies system design and does not require programmable channel sampling devices. Sampling the communications channel produces a sequence of digital samples with each sample corresponding to a snapshot of the communications channel for an instant of time. The sequence of samples can then be correlated, either with a known sequence that is stored in a memory in the receiver or with itself, to detect the presence of a packet. Correlating the sequence of samples with itself is possible if the start of a packet contains a periodic signal since while the communications channel is idle, the sequence of samples will be random and display no pattern, while when a packet is present on the channel, the samples will contain a pattern. It is the presence of a pattern in the samples that can be used to detect the presence of a packet.
Many receivers operate with a sampling rate that is greater than the minimum sampling rate required to accurately recover the transmitted data. The minimum required sampling frequency is twice the frequency of the highest frequency component. This minimum is known as the Nyquist Frequency and is well known to those of ordinary skill in the art of the present invention. However, many receivers operate at a higher sampling rate than the Nyquist Frequency. By operating at higher sampling rates, these receivers are able to more readily separate noise from desired signal (transmitted in a frequency band of interest), making it easier to filter out the desired signal from noise that is in portions of the communications channel outside of the frequency band of interest.
However, operating at a higher sampling rate than necessary results in greater power consumption, hence a correspondingly larger power supply or shorter battery life. Additionally, operating at a higher sampling rate while attempting to detect the presence of a packet on an otherwise idle communications medium has proven to actually degrade the packet detection mechanism. This is due to the fact that sampling at higher sampling rates will also introduce any noise present at those frequencies that are outside of the frequency band of interest into the system often masking the pattern used to detect the presence of a packet.
A need has therefore arisen for a method that will minimize power consumption and at the same time, permit fast and accurate detection of a packet on a communications channel and that introduces a minimum amount of unnecessary noise into the system.